European Patent 0048608 describes a heating concept wherein low voltage electricity is passed through bare uninsulated electrical conductors into a medium which contains a large volume of non-gaseous material for storing the heat for gradual dissipation. The apparatus described in European Application No. 0048608 by Charlton Thermosystems is primarily designed to store and dissipate heat.
The Charlton Thermosystems concept is to heat a medium with an embedded looped heating element. The heating elements are in the form of steel bars, wire rope, and, it is said, can be embedded in a medium or a base material for the medium. Heat is not dissipated uniformly over a given area.
An object of the present invention is to use low voltage electricity to instantaneously heat large surface areas using similar transformation technology to that described in European Patent No. 0049608.
Further objects and advantages of the present invention will become apparent from the ensuing description.
According to the present invention there is provided a method of heating a surface by means of wire mesh or netting heating element spread evenly over or just under the area to be heated through which an electrical current is driven comprising connecting the two ends of the element to the secondary terminals of a transformer which converts the supply voltage to less than 40 volts.
The mesh heating element is elongated and between 10 m and 100 m long, and has a width of between 75 mm and 1200 mm.
Preferably the wire mesh or netting thickness is less than 2 mm.
The heating element can be uninsulated and encased in a settable medium.
The heating element can provide 2500 cm.sup.2 to 6000 cm.sup.2 of surface per 100 watt output for every square meter to be heated.
According to a further aspect of the present invention there is provided a method of heating a surface by means of wire mesh or netting heating element spread evenly over or just under the area to be heated through which an electrical current is driven comprising connecting the two ends of the element to the secondary terminals of a transformer which converts a supply voltage of the order of standard mains voltage volts to less than 40 volts at 16 amps to 240 amps current throughput depending on size.
The mesh heating element may be elongated and may be between 10 m and 100 m long, and have a width of between 75 mm and 1200 mm, depending on the size and dimensions of the area to be heated. Wire thickness may be less than 2 mm diameter which requires only a very thin layer of grout, bedding compound, adhesive or plaster where particular floor surfaces require such a backing.
The voltage across the heating element is preferably less than 40 volts and in most cases will be around 30 volts so the dimension of the copper cable is compatible with the wire in the mesh.
Steel is used as both conductor and heating element and is commonly uninsulated but may be encased in grout, bedding compound, adhesive, plaster or PVC sheet etc to hold the elements in position.
Cross wires may be used to:
1. Conduct heat from hot spots. PA0 2. Conduct electricity around the hot spots. PA0 3. Hold the wires in a uniform layout. PA0 4. Double the heat dissipating surface area of the conductor. PA0 1. The invention uses welded steel mesh to provide a flat heating surface of large area. The thinness of the element, less than 2 mm, and close spaced wire grid 25.4 mm.times.25.4 mm or 19 mm.times.19 mm or 12.7 mm.times.12.7 mm ensures that foot traffic will not detect the heating element and also that no filler or hard covering is required. PA0 2. The invention provides a method of using standard width, wide sheets to achieve the length of circuit with minimum joints. PA0 3. A method for holding elements in position during transport and installation and preventing short circuit is provided where polypropylene or polythene sheet under the element is glued or fused to strip, sheet or patch over the mesh each side of the space between elements and where a cut is made the wires are bent in such a manner that when taped the rows of wire stubs resist any tendency of the rows to move towards each other. PA0 4. Where joints are required the invention provides strong methods of joining with minimal brazing and which avoids fire risk without significantly increasing the thickness of the element. PA0 5. The invention further provides a method of joining the copper cold leads to the mesh without risk of fire or significantly increasing the thickness of the elements by using stranded copper cable with strands of similar dimension to the steel wire in the mesh or flat copper sheet for larger units where the current is very high and which are welded to the mesh in a manner which maintains the integrity of the circuit and the thinness of the heating element. PA0 6. A circuit layout with a plurality of legs is provided, where only one leg of the circuit has to be insulated from the rest to prevent serious short circuit. (Short circuits between two adjacent legs would not have serious consequences).
The method can provide a heating element with some 2500 cm.sup.2 to 6000 cm.sup.2 of surface per 100 watt output for every square meter to be heated.
The heated surface may be a floor, wall and ceiling of a building, yard or vessel.
The special value of the mesh heating is the very even spread of heat over large surface areas, the great strength of the heating element itself which provides security of the system against rough handling during installation, surface medium cracking or movement during use, and the low voltage electricity used, providing electrical safety against contact with the heating element and/or soaking in water.
The invention is specifically aimed for use in providing 24 hour per day switch on/switch off heat to carpet, vinyl, linoleum, tiled and grout or levelling type compound surfacing. The system could be used for many other surface heating requirements and may be used for supplying heat to a surface for only short periods of time if that is what is required.
The mesh proposed to be used is galvanised welded wire mesh constructed of 18 Birmingham wire gauge wire on a square grid 25.4 mm, 19 mm or 12.7 mm centres and the mesh is presently supplied in rolls 915 mm wide by 30.5 m long. However mesh of other dimensions could be used provided the transformer was designed to match the heating element dimensions. (Diamond woven mesh--`chicken netting` can also be used and may even perform better in some situations).
A typical heating element made from this mesh will have a surface area of up to 5760 cm.sup.2 per sq meter in contact with solid medium in which it is embedded. Steel rope or wire method is unsuitable for use right at the surface because the floor would have warm lines at say 30 to 50 cm centres with cold patches between and could damage the surface by means of the uneven heating and cooling. On the other hand the present invention spreads the heat evenly and has the transverse wires to conduct heat laterally and reduce hot spots that might be caused by some material of poor conductivity laying on the heated surface.
By using cross wires in the circuit the heat dissipation is speeded up and the wires will not normally reach more than 40 degrees C. Local overheating along the circuit due to a faulty wire or join would normally produce an increase in resistance and more heat coming into the spot which is already hot. In the system of this invention, the cross wires will conduct electricity around the higher resistance hot patch, putting less power into the spot as well as conducting heat from the area via the wires. This is an important safety feature since the heating system is expected to be used in living rooms and bedrooms where blankets, clothes, rugs etc could overlay the heating surface and is not offered by the technology described in European Application No. 0048608.
Although in most circumstances it is advantageous to have the mesh bare and uninsulated, there are some situations where there could be a requirement to have it insulated such as when the surface to be heated is to be a good conductor of electricity. In these cases the mesh could be plastic coated. In a corrosive environment where the galvanising is vulnerable, it may also be advantageous to have a plastic coating.
Because of the high electrical currents involved in the heating circuit, low resistance at the joining of the conductors is critical to the safety of the equipment. Joints in the heating element for small areas have been minimised by a method of cutting sheets of mesh into a circuit.
Each end of the heating element is normally welded to a copper, aluminium or steel strip or cable of some 15 to 30% the resistivity of the heating element. These are called "cold leads" and each one is in turn bolted or crimped to the transformer terminals in a similar way to how a motor vehicle's electrical starting circuit is connected to the battery. The cold leads are normally insulated.
The transformer that is proposed to be used is any double wound isolating transformer capable of converting the input voltage to under 30 V and safely handling the continuous power consumed.
The transformer primary circuit may be fitted with a thermocouple temperature cut-out and an overload cut-out in case of short circuit. They will also normally have switches connected to various tappings so that one of several heat outputs can be selected. In some situations the input power may be controlled by a thermostat. It will be usual to have an indicator light to show when the heater is on. A rectifier may be used in situations where a direct current is an advantage.
This invention is directed specifically towards heating under carpets in existing buildings and the following features are specific to that end.
The invention/design is such that existing carpet can be lifted and the heating element placed and fixed where required and the carpet re-laid.
The idea of low voltage heating through steel elements has been around for many years but this invention provides the means of spreading the heat more uniformly and more efficiently than any other system.
To our knowledge no other low voltage system has been able to be used directly under the carpet.